Input variable precedence order in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the highest precedence source for Terraform input variables?

Command-line flags (e.g., -var or -var-file) have the highest precedence and override all other sources.

intermediate

List the order of precedence for Terraform input variables from highest to lowest.

1. Command-line flags (-var, -var-file) 2. Environment variables (TF_VAR_name) 3. terraform.tfvars files and other tfvars files 4. Default values in variable blocks

beginner

How do environment variables affect Terraform input variables?

Environment variables prefixed with TF_VAR_ set the value of input variables and override values from tfvars files and defaults, but are overridden by command-line flags.

beginner

What happens if a variable is not set by any source in Terraform?

Terraform uses the default value defined in the variable block. If no default is set and the variable is required, Terraform will prompt the user for input.

intermediate

Why is understanding input variable precedence important in Terraform?

It helps avoid unexpected values by knowing which source will override others, ensuring predictable and controlled infrastructure configuration.

Which source has the lowest precedence for Terraform input variables?

A Environment variables

B Default values in variable blocks

C terraform.tfvars files

D Command-line flags

Default values in variable blocks have the lowest precedence and are used only if no other source provides a value.

If a variable is set in both an environment variable and a terraform.tfvars file, which value does Terraform use?

A Value from terraform.tfvars file

B Value from default in variable block

C Terraform will throw an error

D Value from environment variable

Environment variables override values set in terraform.tfvars files.

How can you override a Terraform variable value without changing any files?

A Pass the value using a command-line flag

B Use a terraform.tfvars file

C Terraform does not allow overriding without files

D Set a default value in the variable block

Passing variables via command-line flags overrides all other sources.

What prefix must environment variables have to set Terraform input variables?

A TF_VAR_

B TF_INPUT_

C TERRAFORM_VAR_

D VAR_

Environment variables must be prefixed with TF_VAR_ followed by the variable name.

If a required variable has no default and is not set by any source, what does Terraform do?

A Uses a null value

B Throws an error and stops

C Prompts the user to enter a value

D Uses an empty string

Terraform prompts the user to input a value for required variables without defaults.

Explain the order in which Terraform applies input variable values from different sources.

Think about which source you would want to override others when running Terraform.

Describe what happens when a Terraform variable is not set anywhere and has no default value.

Consider how Terraform ensures required variables have values.

Practice

1. In Terraform, which source has the highest priority when setting the value of an input variable?

easy

A. Terraform variable definition files (.tfvars)

B. Environment variables

C. Command-line flags (CLI flags)

D. Default values in the variable block

Solution

1. Step 1: Understand Terraform variable precedence

   Terraform uses a specific order to decide variable values: CLI flags first, then environment variables, then .tfvars files, and lastly default values.

2. Step 2: Identify the highest priority source

   Since CLI flags are checked first, they override all other sources.

3. Final Answer:

   Command-line flags (CLI flags) -> Option C

4. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: Remember: CLI flags always override others [OK]

Common Mistakes:

• Thinking environment variables override CLI flags
• Assuming .tfvars files have highest priority
• Confusing default values as highest priority

2. Which of the following is the correct way to pass a variable value via CLI flag when running Terraform?

easy

A. terraform apply -var-file=region=us-west-1

B. terraform apply --set region=us-west-1

C. terraform apply -env region=us-west-1

D. terraform apply -var 'region=us-west-1'

Solution

1. Step 1: Recall CLI flag syntax for variables

   The correct syntax to pass a variable via CLI is using -var followed by 'key=value'.

2. Step 2: Match the correct option

   terraform apply -var 'region=us-west-1' uses -var 'region=us-west-1', which is the correct syntax.

3. Final Answer:

   terraform apply -var 'region=us-west-1' -> Option D

4. Quick Check:

   Use -var 'key=value' for CLI variable input [OK]

Hint: Use -var 'key=value' to pass variables via CLI [OK]

Common Mistakes:

• Using --set instead of -var
• Confusing environment variable syntax with CLI flags
• Using -var-file incorrectly for single variables

3. Given the following Terraform variable declaration and usage:
<pre>variable "env" { default = "dev" } # Command run: # terraform apply -var 'env=prod' # Environment variable: # TF_VAR_env=staging # tfvars file content: # env = "qa" What value will Terraform use for the variable env during this apply?

medium

A. "prod" (from CLI flag)

B. "dev" (default value)

C. "staging" (from environment variable)

D. "qa" (from tfvars file)

Solution

1. Step 1: List variable sources and their precedence

   Terraform checks CLI flags first, then environment variables, then tfvars files, then defaults.

2. Step 2: Identify the highest priority value provided

   CLI flag sets env=prod, which overrides environment variable (staging), tfvars (qa), and default (dev).

3. Final Answer:

   "prod" (from CLI flag) -> Option A

4. Quick Check:

   CLI flag value "prod" is used [OK]

Hint: CLI flags beat env vars and tfvars files [OK]

Common Mistakes:

• Choosing environment variable over CLI flag
• Picking tfvars value over environment variable
• Assuming default value is used when others exist

4. You run Terraform with the environment variable TF_VAR_region=us-east-1 set, but your configuration uses a region variable with a default value of us-west-2. You also have a terraform.tfvars file setting region = "eu-central-1". However, Terraform still uses us-west-2. What is the most likely cause?

medium

A. You passed a CLI flag overriding all other values

B. The environment variable name is incorrect; it should be TF_REGION

C. The variable is marked as sensitive and ignores external values

D. The terraform.tfvars file is not loaded automatically

Solution

1. Step 1: Check environment variable naming

   TF_VAR_region is correct naming for environment variable to set variable 'region'.

2. Step 2: Recall precedence order

   CLI flags > env vars > tfvars > defaults. Env var us-east-1 should override tfvars and default.

3. Step 3: Identify cause of default value

   To override the env var and use default us-west-2, a CLI flag like -var 'region=us-west-2' must have been passed.

4. Final Answer:

   You passed a CLI flag overriding all other values -> Option A

5. Quick Check:

   CLI flags > env vars > tfvars > defaults [OK]

Hint: CLI flags override env vars and tfvars [OK]

Common Mistakes:

• Assuming environment variable name is wrong
• Thinking sensitive variables block external values
• Ignoring possibility of CLI flag override

5. You want to ensure a Terraform variable instance_type always uses the value from a .tfvars file, ignoring any CLI flags or environment variables. Which approach correctly enforces this behavior?

hard

A. Remove default value and only set instance_type in the .tfvars file; avoid passing CLI flags or env vars

B. Terraform cannot enforce this; CLI flags always override .tfvars files

C. Use a local variable to override instance_type with the .tfvars value inside the configuration

D. Use a validation rule in the variable block to reject values not matching the .tfvars file

Solution

1. Step 1: Understand variable precedence limits

   Terraform's precedence is fixed: CLI flags > env vars > .tfvars > defaults. Cannot natively prioritize .tfvars over CLI/env.

2. Step 2: Confirm no enforcement mechanism

   There is no way to access or prioritize the .tfvars value separately from the resolved input variable.

3. Step 3: Why other options fail

   A relies on user discipline; B validation cannot reference .tfvars; C locals use the already-resolved var value from highest precedence.

4. Final Answer:

   Terraform cannot enforce this; CLI flags always override .tfvars files -> Option B

5. Quick Check:

   Precedence fixed, cannot override CLI/env with tfvars [OK]

Hint: Terraform precedence cannot be changed natively [OK]

Common Mistakes:

• Assuming Terraform can block CLI flag precedence natively
• Relying on validation rules to enforce external values
• Thinking removing defaults affects precedence order